I want to see how much time I spend on my computer every day and be able to browse past days' activity levels as well.

I've been using http://www.manictime.com/, which tracks nearly everything I do. But the main data that I find myself using is how much time my computer is active. So while it is a great program, most of its specific data that it's tracking will be unused by me.

I was coming here to firstly suggest manictime, as I used to use this and it is a good all around tracking program. However, this was overkill and I too only wanted to know my active hours, so ditched it for KeyStrokes.

This is a lightweight option which tracks keyboard usage over the day: Keystrokes Homepage.
The only downside is that it does not track mouse movements.

I think Worksnaps (www.worksnaps.net) is a good tool for measuring how much you spent time working on computer. It captures the volume of keystrokes and mouse clicks and accounts the time you interacts with your computer. It can be used for your own use or can be used for managing the remote team by tracking team member's work. All the data is saved on the server (as a SaaS service), therefore you can access it with browser from anywhere. A good tool for productivity and team time tracking.

"Time Doctor" is perfect for you. It records keystrokes being pressed, make screen shots, and prevents wasted time such visiting Facebook or Youtube. I would highly recommend you try it to 2-3 people first and see the result. This has been a proven software to track hours worked and sends reports in a daily/weekly basis. Check its blog at http://blog.timedoctor.com/

I use TimeCamp , it's automatic time tracking software. It's cool because it works basing on keywords so it tracks time automatically and it also stops tracking when I leave my computer and starts when I go back to it.